Estimation of asymmetries between inphase and quadrature branches in multicarrier transmission systems
View Patent ↗Distortions of radio signals transmitted in data blocks in an OFDM method, the distortions being caused by IQ asymmetries, can be estimated and subsequently equalized by means of specific reference data symbols which are transmitted at the transmitter end and may be part of a preamble. The method can be used particularly advantageously in a direct-mixing receiver.
1. A method for the estimation of the distortion of radio signals that is caused by inphase and quadrature (IQ) asymmetries, comprising the step of:
transmitting radio signals in data blocks in a multicarrier transmission method with subcarriers n and subcarriers −n, wherein in the transmission spectrum subcarriers n and −n are arranged symmetrically with respect to a center frequency fc, which data blocks each contain a number of data symbols d composed of subcarrier symbols d n and d −n , wherein
each data block contains two reference data symbols that succeed one another at instants i, i−1,
during the reference data symbols with regard to each subcarrier pair n/−n, the transmitted data symbol on one subcarrier remains temporally constant, while that on the respective other subcarrier changes sign, and wherein
the distortion parameters are calculated from the received subcarrier symbols.
2. The method as claimed in claim 1 , wherein
the distortion is modeled by the equation
[
d
^
n
′
(
i
)
d
^
-
n
′
*
(
i
)
]
=
[
a
n
RX
b
n
RX
b
-
n
RX
*
a
-
n
RX
*
]
︸
A
RX
·
[
C
n
0
0
C
-
n
*
]
︸
C
·
[
a
n
TX
b
n
TX
b
-
n
TX
*
a
-
n
TX
*
]
︸
A
TX
·
[
d
n
(
i
)
d
-
n
*
(
i
)
]
=
[
a
n
b
n
b
-
n
*
a
-
n
*
]
︸
A
·
[
d
b
(
i
)
d
-
n
*
(
i
)
]
wherein {circumflex over (d)}′ n (i) are the distorted symbols received at the instant i on the subcarrier n, d n (i) are the undistorted transmitted symbols, A TX is the transmitter-end IQ distortion matrix, A RX is the reception-end IQ distortion matrix, and C is the distortion matrix of the multipath channel.
3. The method as claimed in claim 2 , wherein
assuming d n (i)=−d n (i−1) and d −n (i)=d −n (i−1) for all subcarrier pairs n/−n and a channel that is approximately constant for two data symbols that succeed one another, the distortion parameters of the matrix A are calculated as
a
n
=
0.5
·
d
^
n
′
(
i
)
-
d
^
n
′
(
i
-
1
)
d
n
(
i
)
b
n
=
0.5
·
d
^
n
′
(
i
)
+
d
^
n
′
(
i
-
1
)
d
-
n
*
(
i
)
b
-
n
=
0.5
·
d
^
-
n
′
(
i
)
-
d
^
-
n
′
(
i
-
1
)
d
n
*
(
i
)
a
-
n
=
0.5
·
d
^
-
n
′
(
i
)
+
d
^
-
n
′
(
i
-
1
)
d
-
n
(
i
)
.
4. The method as claimed in claim 2 , wherein
assuming d n (i)=d n (i−1) and d −n (i)=−d −n (i−1) for all subcarrier pairs n/−n and a channel that is approximately constant for two data symbols that succeed one another, the distortion parameters of the matrix A are calculated as
a
n
=
0.5
·
d
^
n
′
(
i
)
+
d
^
n
′
(
i
-
1
)
d
n
(
i
)
b
n
=
0.5
·
d
^
n
′
(
i
)
-
d
^
n
′
(
i
-
1
)
d
-
n
*
(
i
)
b
-
n
=
0.5
·
d
^
-
n
′
(
i
)
+
d
^
-
n
′
(
i
-
1
)
d
n
*
(
i
)
a
-
n
=
0.5
·
d
^
-
n
′
(
i
)
-
d
^
-
n
′
(
i
-
1
)
d
-
n
(
i
)
.
5. The method as claimed in claim 2 , wherein
for the case where d n (i)=−d n (i−1) and d −n (i)=d −n (i−1) for all subcarrier pairs n/−n and only a transmitter-end IQ asymmetry is present, the distortion parameters are calculated as
b
n
TX
=
d
^
n
′
(
i
)
+
d
^
n
′
(
i
-
1
)
d
^
n
′
(
i
)
-
d
^
n
′
(
i
-
1
)
·
d
n
(
i
)
d
-
n
*
(
i
)
b
-
n
TX
=
d
^
-
n
′
(
i
)
-
d
^
-
n
′
(
i
-
1
)
d
^
-
n
′
(
i
)
+
d
^
-
n
′
(
i
-
1
)
·
d
-
n
(
i
)
d
n
*
(
i
)
.
6. The method as claimed in claim 2 , wherein
for the case where d n (i)=d n (i−1) and d −n (i)=−d −n (i−1) for all subcarrier pairs n/−n and only a transmitter-end IQ asymmetry is present, the distortion parameters are calculated as
b
n
TX
=
d
^
n
′
(
i
)
-
d
^
n
′
(
i
-
1
)
d
^
n
′
(
i
)
+
d
^
n
′
(
i
-
1
)
·
d
n
(
i
)
d
-
n
*
(
i
)
b
-
n
TX
=
d
^
-
n
′
(
i
)
+
d
^
-
n
′
(
i
-
1
)
d
^
-
n
′
(
i
)
-
d
^
-
n
′
(
i
-
1
)
·
d
-
n
(
i
)
d
n
*
(
i
)
.
(
6.2
)
7. The method as claimed in claim 1 , wherein
the reference data symbols are transmitted as part of a preamble of the data block.
8. The method as claimed in claim 1 , wherein
the multicarrier transmission method is an orthogonal frequency division multiplexing (OFDM) method.
9. The method as claimed in claim 1 , wherein
the reference data symbols are modulated according to a BPSK or D-BPSK modulation method.
10. A method for the equalization of the distortion of mobile radio signals that is caused by IQ asymmetries, comprising a method for estimation according to claim 1 , the method for equalization comprising the steps of:
determining transmitted payload-data data symbols from received payload-data data symbols from said data blocks, and
calculating the distortion parameters from the received subcarrier symbols.
11. The method of claim 1 wherein the method is used in a direct-mixing receiver.
12. The method of claim 1 wherein the method is used in a heterodyne receiver with a direct-mixing second stage.
13. A system for the estimation of the distortion of radio signals that is caused by inphase and quadrature (IQ) asymmetries, comprising:
means for transmitting radio signals in data blocks in a multicarrier transmission method with subcarriers n and subcarriers −n, wherein in the transmission spectrum subcarriers n and −n are arranged symmetrically with respect to a center frequency fc, which data blocks each contain a number of data symbols d composed of subcarrier symbols d n and d −n ,
wherein each data block contains two reference data symbols that succeed one another at instants i, i−1,
wherein during the reference data symbols with regard to each subcarrier pair n/−n, the transmitted data symbol on one subcarrier remains temporally constant, while that on the respective other subcarrier changes sign, and
means for calculating the distortion parameters from the received subcarrier symbols.
14. The system as claimed in claim 13 , comprising means for modeling the distortion by the equation
[
d
^
n
′
(
i
)
d
^
-
n
′
*
(
i
)
]
=
[
a
n
RX
b
n
RX
b
-
n
RX
*
a
-
n
RX
*
]
︸
A
RX
·
[
C
n
0
0
C
-
n
*
]
︸
C
·
[
a
n
TX
b
n
TX
b
-
n
TX
*
a
-
n
TX
*
]
︸
A
TX
·
[
d
n
(
i
)
d
-
n
*
(
i
)
]
=
[
a
n
b
n
b
-
n
*
a
-
n
*
]
︸
A
·
[
d
b
(
i
)
d
-
n
*
(
i
)
]
wherein {circumflex over (d)}′ n (i) are the distorted symbols received at the instant i on the subcarrier n, d n (i) are the undistorted transmitted symbols, A TX is the transmitter-end IQ distortion matrix, A RX is the reception-end IQ distortion matrix, and C is the distortion matrix of the multipath channel.
15. The system as claimed in claim 14 , comprising means for calculating, assuming d n (i)=−d n (i−1) and d −n (i)=d −n (i−1) for all subcarrier pairs n/−n and a channel that is approximately constant for two data symbols that succeed one another, the distortion parameters of the matrix A as:
a
n
=
0.5
·
d
^
n
′
(
i
)
-
d
^
n
′
(
i
-
1
)
d
n
(
i
)
b
n
=
0.5
·
d
^
n
′
(
i
)
+
d
^
n
′
(
i
-
1
)
d
-
n
*
(
i
)
b
-
n
=
0.5
·
d
^
-
n
′
(
i
)
-
d
^
-
n
′
(
i
-
1
)
d
n
*
(
i
)
a
-
n
=
0.5
·
d
^
-
n
′
(
i
)
+
d
^
-
n
′
(
i
-
1
)
d
-
n
(
i
)
.
16. The system as claimed in claim 14 , comprising means for calculating, assuming d n (i)=d n (i−1) and d −n (i)=−d −n (i−1) for all subcarrier pairs n/−n and a channel that is approximately constant for two data symbols that succeed one another, the distortion parameters of the matrix A as:
a
n
=
0.5
·
d
^
n
′
(
i
)
+
d
^
n
′
(
i
-
1
)
d
n
(
i
)
b
n
=
0.5
·
d
^
n
′
(
i
)
-
d
^
n
′
(
i
-
1
)
d
-
n
*
(
i
)
b
-
n
=
0.5
·
d
^
-
n
′
(
i
)
+
d
^
-
n
′
(
i
-
1
)
d
n
*
(
i
)
a
-
n
=
0.5
·
d
^
-
n
′
(
i
)
-
d
^
-
n
′
(
i
-
1
)
d
-
n
(
i
)
.
17. The system as claimed in claim 14 , comprising means for calculating, for the case where d n (i)=−d n (i−1) and d −n (i)=d −n (i−1) for all subcarrier pairs n/−n and only a transmitter-end IQ asymmetry is present, the distortion parameters as:
b
n
TX
=
d
^
n
′
(
i
)
+
d
^
n
′
(
i
-
1
)
d
^
n
′
(
i
)
-
d
^
n
′
(
i
-
1
)
·
d
n
(
i
)
d
-
n
*
(
i
)
b
-
n
TX
=
d
^
-
n
′
(
i
)
-
d
^
-
n
′
(
i
-
1
)
d
^
-
n
′
(
i
)
+
d
^
-
n
′
(
i
-
1
)
·
d
-
n
(
i
)
d
n
*
(
i
)
.
18. The system as claimed in claim 14 , comprising means for calculating, for the case where d n (i)=d n (i−1) and d −n (i)=−d −n (i−1) for all subcarrier pairs n/−n and only a transmitter-end IQ asymmetry is present, the distortion parameters as:
b
n
TX
=
d
^
n
′
(
i
)
-
d
^
n
′
(
i
-
1
)
d
^
n
′
(
i
)
+
d
^
n
′
(
i
-
1
)
·
d
n
(
i
)
d
-
n
*
(
i
)
b
-
n
TX
=
d
^
-
n
′
(
i
)
+
d
^
-
n
′
(
i
-
1
)
d
^
-
n
′
(
i
)
-
d
^
-
n
′
(
i
-
1
)
·
d
-
n
(
i
)
d
n
*
(
i
)
.
(
6.2
)
19. The system as claimed in claim 13 , comprising means for transmitting the reference data symbols as part of a preamble of the data block.
20. The system as claimed in claim 13 , wherein
the multicarrier transmission method is an orthogonal frequency division multiplexing (OFDM) method.
21. The system as claimed in claim 13 , wherein
the reference data symbols are modulated according to a BPSK or D-BPSK modulation method.